1. Field of the Invention
The present invention relates to a protective element for protecting a circuit to be protected from an overcurrent or an overvoltage by tripping of a PTC element.
2. Description of the Related Art
Conventionally, current fuses that are made of a low-melting-point metal, such as lead, tin or antimony that are heated and melted off by overcurrents, are known as protective elements for interrupting overcurrents to circuits to be protected.
Moreover, PTC (Positive Temperature Coefficient) elements are known as elements that, when overcurrent flows, generate heat, thereby increasing their resistance and limiting the current flowing through the circuit to be protected.
Furthermore, protective devices have been proposed, in which such a protective element is combined with a voltage sensing means, so that the circuit to be protected is protected not only from overcurrent but also from overvoltages.
FIG. 9 is a circuit diagram of such a protective device (see Japanese Patent Application Laid-Open No. 8-236305). In this circuit, the terminals A1 and A2 are connected to the electrode terminals of the device to be protected, such as for example a lithium-ion battery, and the terminals B1 and B2 are connected to the electrode terminals of, for example, a charging device.
In the protective element 20X used for the protective device in FIG. 9, a PTC element 1 and a low-melting-point metal member 2 are connected in series between a first terminal xe2x80x9caxe2x80x9d and a second terminal xe2x80x9cbxe2x80x9d of the protective element 20X, and a heat-generating member 3 is connected between a third terminal xe2x80x9ccxe2x80x9d and an electrode 5d. Moreover, this protective device is provided with a Zehner diode and a transistor as a voltage sensing means and a switching means.
With this protective device, under normal conditions, current does not flow through the heat-generating member 3, and the PTC element 1 and the low-melting-point metal member 2 form a conduction path between the terminals of the device to be protected, such as a lithium-ion battery, and the electrode terminals of the charging device or the like. However, but when a reverse voltage that is larger than a predetermined breakdown voltage is applied to the Zehner diode, an abrupt base current ib flows, which causes a large collector current ic to flow through the heat-generating member 3 and heat the heat-generating member 3, the low-melting-point metal member 2 that is at a position near the heat-generating member 3 melts off, and the continuation of an overcharge in the device to be protected, such as a lithium-ion battery connected to the terminals A1 and A2, can be prevented.
On the other hand, when an overcurrent exceeding a certain value flows between the terminals A1 and B1, first, the PTC element 1 limits the current, then the low-melting-point metal member 2 melts off and interrupts the current.
In the protective element 20X used for the protective device as shown in FIG. 9, electrodes 5a, 5b, 5c and 5d are formed on a substrate 4, as shown in a top view by FIG. 10A and a cross-sectional view by FIG. 10B. The heat-generating member 3 is formed between the electrode 5c and the electrode 5d, and is covered with an insulating layer 6. The PTC element 1 is layered on the electrode 5a, the low-melting-point metal member 2 is formed extending over the PTC element 1, the electrode 5b and the electrode 5d on the substrate 4, and these parts are covered by a protective cap 7.
Thus, there is the problem that there is a limit to how much the height h of the protective element 20X can be reduced, so that it is not possible to make the device thinner. Moreover, the number of parts is great, so that there is the problem that it is difficult to reduce the manufacturing costs.
It is thus an object of the present invention to overcome the problems of the related art, and to provide a protective element using a PTC element, which can be manufactured easily, with fewer components, and at lower costs, and which can be made thinner.
The inventors of the present invention have found out that when the protective element is made of (i) a first PTC element and (ii) a low-melting-point metal member, a heat-generating member or a second PTC element, then the substrate, which hitherto used to be necessary for mounting the protective element on, becomes unnecessary, and the entire protective element can be made thinner, if a PTC material constituting (i) the first PTC element serves as a substrate for the (ii) low-melting-point metal member, the heat-generating member or the second PTC element, thus conceiving the protective element of the present invention.
In other words, the present invention provides a protective element comprising: a first PTC element; and a low-melting-point metal member, a heat-generating member or a second PTC element, where a PTC material constituting the first PTC element serves as a substrate for a low-melting-point metal member, a heat-generating member or a second PTC element.